This application claims the benefit of Korean Application No. 2000-24481 filed on May 8, 2000, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a transflective liquid crystal display and method of fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for a high contrast ratio.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) is classified as a transmission type and a reflection type depending on implementing an internal or external light source. The transmission type has a liquid crystal display panel, which does not emit light itself, and has a backlight as a light-illuminating section.
The backlight is disposed at the rear or one side of the panel. The amount of the light from the backlight that passes through the liquid crystal panel is controlled by the liquid crystal panel in order to implement an image display. In other words, the light from the backlight varies and displays images according to the arrangement of the liquid crystal molecules. However, the backlight of the transmission type LCD consumes 50% or more of the total power consumed by the LCD device. Providing a backlight therefore increases power consumption.
In order to overcome the above problem, a reflection type LCD has been selected for portable information apparatuses that are often used outdoors or carried with users. Such a reflection type LCD is provided with a reflector formed on one of a pair of substrates. Thus, ambient light is reflected from the surface of the reflector. The reflection type LCD using the reflection of ambient light is disadvantageous in that a visibility of the display is extremely poor when surrounding environment is dark.
In order to overcome the above problems, a construction which realizes both a transmissive mode display and a reflective mode display in one liquid crystal display device has been proposed. This is so called a transflective liquid crystal display device. The transflective liquid crystal display (LCD) device alternatively acts as a transmissive LCD device and a reflective LCD device. Due to the fact that a transflective LCD device can make use of both internal and external light sources, it can be operated in bright ambient light as well as has a low power consumption.
FIG. 1 shows a typical transflective liquid crystal display (LCD) device 11. The transflective LCD device 11 includes upper and lower substrates 15 and 21 with an interposed liquid crystal 23. The upper and lower substrates 15 and 21 are sometimes respectively referred to as a color filter substrate and an array substrate.
On the surface facing into the lower substrate 21, the upper substrate 15 includes a black matrix 16 and a color filter layer 17. The color filter layer 17 includes a matrix array of red (R), green (G), and blue (B) color filters that are formed, such that each color filter is divided by the black matrix 16. The upper substrate 15 also includes a common electrode 13 over the color filter layer 17 and the black matrix 16.
On the surface facing into the upper substrate 15, the lower substrate 21 includes an array of thin film transistors (designated as TFT xe2x80x9cTxe2x80x9d in FIG. 1) that act as switching devices. The array of thin film transistors is formed to correspond to the matrix of color filters. A plurality of gate and data lines 25 and 27 are positioned and crossed over each other. A TFT is located near at each crossing portion of the gate and data lines 25 and 27. The lower substrate 21 also includes a plurality of pixel electrodes 19 in the area between the gate and data lines 25 and 27. Such an area is often referred to as pixel regions xe2x80x9cPxe2x80x9d, as shown in FIG. 1.
Each pixel electrode 19 includes a transparent portion 19a and a reflective portion 19b. The transparent portion 19a is usually formed of a transparent conductive material having good light transmissivity, such as indium tin oxide (ITO). Alternatively, the transparent portion 19a may be a hole. Moreover, a conductive metallic material having a superior light reflectivity is used for the reflective portion 19b. 
FIG. 2, a schematic cross-sectional view of a transflective LCD device 57 illustrating an operation of such devices. For convenience, the color filters 17 (shown in FIG. 1) are not shown in FIG. 2 because it does not affect the polarization state of light. As shown in FIG. 2, the transflective LCD device 57 includes lower and upper substrates 21 and 15 and an liquid crystal layer 23 having optical anisotropy is interposed therebetween.
The upper substrate 15 includes a common electrode 13 on its surface facing into the lower substrate 21. On the other surface of the upper substrate 15, an upper quarter wave plate (QWP) 45 (often referred to as a retardation film), which has a phase difference xcex/4, and an upper polarizer 55 are formed in series.
The lower substrate 21 includes a transparent electrode 50 on its surface facing into the upper substrate 15. A passivation layer 48 and a reflective electrode 19b are formed in series on the transparent electrode 50. The reflective electrode 19b and the transparent electrode 50 act together as a pixel electrode (the reference numeral 19 of FIG. 1). The passivation layer 48 and the reflective electrode 19b also have a transmitting hole 19a. 
Various configurations and structures may be implemented for the pixel electrode in the transflective LCD device. However, the passivation layer 48 should be formed between the transparent electrode 50 and the reflective electrode 19b. 
In order to form a pixel electrode, a transparent conductive material such as ITO (indium tin oxide) or IZO (indium zinc oxide) is deposited on the lower substrate 21 and then patterned into the transparent electrode 50.
Next, the passivation layer 48 is formed on the transparent electrode 50. The conductive metallic material having superior reflectivity, such as aluminum (Al) or the like, is deposited on the passivation layer 48 and then patterned to form a reflective electrode 19b. In this patterning process, the transmitting hole 19a as a transparent portion is formed at the central portion of the reflective electrode 19b. Moreover the central portion of the passivation layer 48 corresponding to the hole 19a is also patterned to expose the central portion of the transparent electrode 50.
Accordingly, the transparent electrode 50 and the reflective electrode 19b serve as a pixel electrode. Moreover, this structure makes different cell gaps xe2x80x9cd1xe2x80x9d and xe2x80x9cd2xe2x80x9d between the common electrode 13 and the pixel electrode (the reflective electrode 19b and the transparent electrode 50). xe2x80x9cd1xe2x80x9d denotes the first cell gap between the common electrode 13 and the reflective electrode 19b while xe2x80x9cd2xe2x80x9d denotes the second cell gap between the common electrode 13 and the transparent electrode 50.
On the other surface of the lower substrate 21, a lower quarter wave plate 54 and a lower polarizer 52 are formed in series. Moreover, a backlight device 41 is arranged below the lower polarizer 52.
In a homogeneous liquid crystal or twisted nematic (TN), its molecules are oriented in the vertical direction when a voltage is applied (Von=5V) and used as a liquid crystal layer 23. When an optical retardation xe2x80x9cxcex94nxc2x7d1xe2x80x9d of a first cell gap is xcex/4 (xcex=550 nm) and a second cell gap xe2x80x9cd2xe2x80x9d is twice as large as the first cell gap xe2x80x9cd1xe2x80x9d as described by equations (1) and (2), an optical retardation xe2x80x9cxcex94nxc2x7d2xe2x80x9d of the second cell gap xe2x80x9cd2xe2x80x9d is shown in equation (3).
xcex94nxc2x7d1=xcex/4xe2x80x83xe2x80x83(1) 
d2≅2d1xe2x80x83xe2x80x83(2) 
∴xcex94nxc2x7d2≅xcex/2xe2x80x83xe2x80x83(3) 
In the above equations, xcex94n is birefringence, d1 denotes the first cell gap between the reflective electrode and the common electrode, d2 denotes the second cell gap between the transparent electrode and the common electrode. xcex is the wavelength of the light, and xcex/4 is a phase shift value of the light when the light passes through a reflective portion of the liquid crystal layer 23 between the common electrode 13 and the reflective electrode 19b at once. xcex/2 is a phase shift value of the light when the light passes through a transparent portion of the liquid crystal layer between the common electrode 13 and the transparent electrode 50 at once.
Accordingly, the optical retardation xe2x80x9cxcex94nxc2x7d2xe2x80x9d of the second cell gap xe2x80x9cd2xe2x80x9d, as shown by equation (3), is xcex/2 (xcex=550 nm). In the reflective mode, the ambient light passes through the liquid crystal layer 23 twice, i.e., as the ambient light is reflected by the reflective electrode 19b. 
As mentioned above, since different cell gaps (the transparent portion and the reflective portion) are formed in the liquid crystal layer 23, there is no difference in the optical retardation of light passing both through the transparent portion and through the reflective portion.
FIG. 3 shows a liquid crystal orientation in cases that the voltage is applied and not applied. As shown, molecules of the liquid crystal layer 23 are arranged in the horizontal direction along the upper and lower substrates 13 and 21 when the voltage is not applied. On the other hand, the molecules are arranged in the vertical direction perpendicular to the upper and lower substrates 13 and 21 when the voltage is applied. However, in the ON-state, the molecules close to the upper and lower substrate 13 and 21 are not oriented properly because of an anchoring energy generated between the liquid crystal molecules and each substrate.
Therefore, the liquid crystal layer 23 derives characteristics of birefringence because the liquid crystal molecules are not properly oriented. Namely, a residual optical phase retardation can exist because of unchanged orientation or alignment of some of the liquid crystal molecules that are close to the upper and lower substrates 13 and 21. These cause the light leakage in a dark state of the LCD device.
In general, in case of the TN liquid crystal that has a twisted angle of 90xc2x0, molecules detached from the upper and lower substrates are mostly arranged perpendicular to the pair of substrates when the voltage is applied since these molecules are not affected from the anchoring energy. Moreover, the molecules close to the pair of substrates are not arranged in the vertical direction. Thus, the orientation direction of the TN liquid crystal molecules close to the upper substrate are arranged perpendicular to that of the molecules close to the lower substrate. As a result, an optical effect of the TN liquid crystal is offset each other.
However, in case of the homogeneous liquid crystal that has a twisted angle of 0xc2x0 as shown in FIG. 3, these molecules close to the upper and lower substrates 13 and 21 affect the optical effect of the liquid crystal layer 23. This is because an orientation direction of the molecules located close to the upper substrate 13 are parallel to that of the molecules around the lower substrate 23.
Therefore, a light leakage occurs in the dark state of the LCD device when the upper retardation film (the reference numeral 45 of FIG. 2) and the lower retardation film (the reference numeral 54 of FIG. 2) have the same phase difference value. In addition, a contrast ratio of the transflective LCD device is deteriorated by the light leakage.
FIG. 4 is a simplified cross-sectional view in order to calculate a phase retardation value of the above-mentioned homogeneous liquid crystal. As shown, the upper polarizer 55 and the lower polarizer 52 are facing into each other. The liquid crystal layer 23 that has the optical retardation xcex/2 is interposed between the pair of polarizers 55 and 52. Thereafter, a transmittance is measured by a simulator such as an LCD master.
FIG. 5 is a graph illustrating a transmittance when a voltage is applied to the transflective LCD device of FIG. 4. When the voltage is applied, i.e., the TFT is turned ON, the transmittance should be ideally zero (i.e., T=0). However, the transmittance results in 0.038 (i.e., T=0.038) in experiment. Moreover, the transmittance can be calculated by the following equation (4).                     T        =                              Sin            2                    ⁢          2          ⁢                                    φSin              2                        ⁡                          [                                                                    π                    ·                    Δ                                    ⁢                                      xe2x80x83                                    ⁢                                      n                    ·                    d                                                  λ                            ]                                                          (        4        )            
In equation (4), xe2x80x9cTxe2x80x9d denotes a transmittance, xe2x80x9cxcex94nxc2x7dxe2x80x9d denotes an optical retardation, xcex denotes a wavelength of light, xcfx86 denotes an angle between a tranmissive axis of the polarizer and an optical axis of the liquid crystal layer. From the above equation (4), the optical retardation xe2x80x9cxcex94nxc2x7dxe2x80x9d is 34 nm (i.e., xcex94nxc2x7d=34 nm), when xcex is 550 nm and xcfx86 is 45 degrees.
Accordingly, the light passing through the liquid crystal layer, which includes a homogeneous liquid crystal, has an optical retardation when a voltage is applied to the related art transflective LCD device. Thus, a complete dark state can not be achieved because of the light leakage described above.
Accordingly, the present invention is directed to a transflective liquid crystal display and a method of fabricating the same that substantially obviate one or more of the problems due to limitations and disadvantages of the related art.
An object of the invention is to provide a transflective LCD display and a method of fabricating the same achieving a high contrast ratio.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a transflective liquid crystal display includes upper and lower substrates facing into and spaced apart from each other, wherein the upper and lower substrates include a plurality of pixel regions that display images, a liquid crystal layer interposed between the upper and lower substrates, wherein the liquid crystal layer has a first adjusted thickness to compensate an residual optical retardation of incident light caused by anchored liquid crystals near an alignment layer when a maximum operation voltage is applied, an upper quarter wave plate (QWP) on the upper substrate, wherein the upper quarter wave plate has a second adjusted thickness to compensate the residual optical retardation caused by the liquid crystal layer when the maximum operation voltage is applied, an upper polarizer on the upper quarter wave plate, a transparent common electrode on a surface of the upper substrate facing into the lower substrate, a pixel electrode over a first surface of the lower substrate, wherein the pixel electrode corresponds to each pixel region, and the pixel electrode is divided into transparent and reflective portions, a lower quarter wave plate (QWP) on a second surface of the lower substrate, a lower polarizer below the lower quarter wave plate, and a backlight device arranged to be adjacent to the lower polarizer.
The liquid crystal layer has a thickness of xe2x80x9cdxe2x80x9d and a transmittance of xe2x80x9cTxe2x80x9d when a maximu voltage for operating is applied, wherein an adjusted thickness of liquid crystal layer is calculated using the following equation:       T    =                  Sin        2            ⁢      2      ⁢                        φSin          2                ⁡                  [                                                    π                ·                Δ                            ⁢                              xe2x80x83                            ⁢                              n                ·                                  d                  1                                                      λ                    ]                      ,
where, T is equals to the value of the transmittance, xcfx86 is an angle between an optical axis of the liquid crystal layer and a transmissive axis of the polarizer, xcex94n is a birefringence of the liquid crystal layer, and wherein d1 is calculated from the above equation, and the liquid crystal layer then has the adjusted thickness xe2x80x9cd+d1xe2x80x9d for compensating the optical retardation. A value of xcfx86 mentioned above is 45 degrees.
The upper QWP has a thickness of xe2x80x9cdxe2x80x9d and a transmittance of xe2x80x9cTxe2x80x9d, wherein an adjusted thickness of the upper QWP for compensation is calculated using the following equation:       T    =                  Sin        2            ⁢      2      ⁢                        φSin          2                ⁡                  [                                                    π                ·                Δ                            ⁢                              xe2x80x83                            ⁢                              n                ·                                  d                  2                                                      λ                    ]                      ,
where, T is equals to the value of the transmittance, xcfx86 is an angle between a slow axis of the upper QWP and a transmissive axis of the polarizer, xcex94n is a birefringence of the upper QWP, and wherein d2 is calculated from the above equation and the upper QWP then has the adjusted thickness xe2x80x9cd+d2xe2x80x9d for compensating the optical retardation. A value of xcfx86 mentioned above is 45 degrees. In accordance with the purpose of the invention, in another aspect, the principles of the present invention provide a transflective liquid crystal display device, including: upper and lower substrates facing and spaced apart from each other, wherein the upper and lower substrates include a plurality of pixel region that display images; a liquid crystal layer interposed between the upper and lower substrates, wherein the liquid crystal layer has an adjusted thickness of compensating a residual optical retardation of incident light caused by anchored liquid crystals near an alignment layer when a maximum voltage for operating is applied; a first upper retardation film over the upper substrate; a second upper retardation film between the first upper retardation film and the upper substrate, wherein the second upper retardation film has an adjusted thickness of compensating an optical retardation caused by the liquid crystal layer; an upper polarizer on the first upper retardation film; a transparent common electrode on a surface of the upper substrate facing the lower substrate; a transparent electrode on a surface of the lower substrate facing the upper substrate; a pixel electrode over the lower substrate, wherein the pixel electrode corresponds to each pixel region, and wherein the pixel electrode is divided into transparent and reflective portions; a second lower retardation film on the other surface of the lower substrate, wherein the second lower retardation film has an adjusted thickness of compensating a residual optical retardation caused by the liquid crystal layer when a maximum voltage for operating is applied; a first lower retardation film under the second lower retardation film; a lower polarizer under the first lower retardation film; and a backlight device arranged adjacent to the lower polarizer.
The transparent portion of the pixel electrode includes a transparent electrode, and wherein the transparent electrode is disposed on a surface of the lower substrate facing the upper substrate. The transflective liquid crystal display device further comprises a passivation layer on the transparent electrode, the passivation layer having a transmitting hole in its central portion. Moreover, the reflective portion of the pixel electrode includes a reflective electrode, which is disposed on the passivation layer and has the transmitting hole in its central portion.
The transflective liquid crystal display device further includes a passivation layer on the transparent electrode, and the passivation layer has a transmitting hole in its central portion. Moreover, the transflective liquid crystal display device further includes a reflective electrode on the passivation layer, and the reflective electrode has the transmitting hole in its central portion.
The liquid crystal layer has a thickness of xe2x80x9cdxe2x80x9d in a reflective portion in order for an optical retardation of xcex/4, and a thickness of xe2x80x9c2dxe2x80x9d in a transmissive portion in order ofr an optical retardation of xcex/2, wherein the liquid crystal layer has a transmittance of xe2x80x9cTxe2x80x9d when a maximum voltage for operating is applied, wherein an adjusted thickness of the liquid crystal layer is calculated using the following equation:       T    =                  Sin        2            ⁢      2      ⁢                        φSin          2                ⁡                  [                                                    π                ·                Δ                            ⁢                              xe2x80x83                            ⁢                              n                ·                                  d                  *                                                      λ                    ]                      ,
where, T is equals to the value of the transmittance, xcfx86 is an angle between an optical axis of the liquid crystal layer and a transmissive axis of the polarizer, xcex94n is a birefringence of the liquid crystal layer, and d* is a thickness of the liquid crystal layer, wherin d* is d1 or d2, wherein d1 and d2 are calculated from the above equation, where, d1 is a first auxiliary thickness of the liquid crystal layer when the residual optical retardation of the light is xcex3 in the reflective portion, d2 is a second auxiliary thickness of the liquid crystal layer when the residual optical retardation of the light is xcfx89 in the transmissive portion, and then the phase difference between the transmissive and reflective portions is xcex4=xcfx89xe2x88x92xcex3, and wherein the reflective portion of the liquid crystal layer has the adjusted thickness of xe2x80x9cd+d1xe2x80x9d and the transmissive portion of the liquid crystal display has the adjusted thickness of xe2x80x9c2d+d2xe2x80x9d for compensating the optical retardation. A value of xcfx86 mentioned above is 45 degrees.
The second upper retardation film has a thickness of xe2x80x9cd4xe2x80x9d and a transmittance of xe2x80x9cTxe2x80x9d, wherein an adjusted thickness of the second retardation film is calculated using the following equation:       T    =                  Sin        2            ⁢      2      ⁢                        φSin          2                ⁡                  [                                                    π                ·                Δ                            ⁢                              xe2x80x83                            ⁢                              n                ·                                  d                  *                                                      λ                    ]                      ,
where, T is equals to the value of the transmittance, xcfx86 is an angle between a slow axis of the retardation film and a transmissive axis of the polarizer, xcex94n is a birefringence of the retardation film, and d* is a thickness of the liquid crystal layer, wherin d* is d1, d2 or d3, wherein d1, d2 and d3 are calculated from the above equation and the following equation:
d2(xcfx89)=d1(xcex3)+d3(xcex4), 
where, d1 is a first auxiliary thickness of the liquid crystal layer when the residual optical retardation of the light is xcex3 in the reflective portion, d2 is a second auxiliary thickness of the liquid crystal layer when the residual optical retardation of the light is xcex4 in the transmissive portion, and then the phase difference between the transmissive and reflective portions is xcex4=xcfx89xe2x88x92xcex3, and wherein the second upper retardation film has the thickness of xe2x80x9cd4+d1(xcex3)xe2x80x9d for compensating the optical retardation. A value of xcfx86 mentioned above is 45 degrees.
The second lower retardation film has the thickness of xe2x80x9cd4xe2x88x92d3(xcex4)xe2x80x9d for compensating the optical retardation. The first upper and lower retardation films are beneficially half wave plates (HWPs) and the second upper and lower retardation films are beneficially quarter wave plates (QWPs). The transmissive axis of the lower polarizer is perpendicular to that of the upper polarizer. The slow axis of the first upper retardation film is perpendicular to that of the first lower retardation film. The slow axis of the second upper retardation film is perpendicular to that of the second lower retardation film. The optical axis of the liquid crystal layer is parallel with the slow axis of the second lower retardation film.
In another aspect of the present invention, a transflective liquid crystal display device includes upper and lower substrates facing and spaced apart from each other, wherein the upper and lower substrates include a plurality of pixel region that display images; an upper quarter wave plate (QWP) on the upper substrate; an upper polarizer on the upper quarter wave plate; a lower quarter wave plate (QWP) under the lower substrate; a lower polarizer under the lower quarter wave plate; a backlight device arranged adjacent to the lower polarizer; a liquid crystal layer interposed between the upper and lower substrates; a transparent common electrode on a surface of the upper substrate facing the lower substrate; an upper alignment layer between the transparent common electrode and the liquid crystal layer; a pixel electrode over the lower substrate, wherein the pixel electrode corresponds to each pixel region, and wherein the pixel electrode is divided into transparent and reflective portions; and a lower alignment layer between the pixel electrode and the liquid crystal layer; wherein a transmissive axis of the upper polarizer is perpendicular to a transmissive axis of the lower polarizer; wherein a slow axis of the upper QWP is perpendicular to a slow axis of the lower QWP; wherein the slow axis of the upper QWP forms an angle of 45xc2x0 with the transmissive axis of the upper polarizer; wherein an optical retardation of the upper QWP is xcex/4+xcex1; wherein xcex1 ranges from zero to 100 nm; and wherein the slow axis of the lower QWP is parallel with an orientation direction of the liquid crystal display layer.
An optical retardation of the liquid crystal layer is xcex/4+xcex1. The optical retardation of the liquid crystal layer is different between transmissive and reflective portions, wherein the optical retardation is xcex/4+xcex1in the reflective portion, and wherein the optical retardation is xcex/2+xcex2in the transmissive portion, and wherein xcex2 ranges from zero to 100 nm. Here, an optimum value of xcex1 for adjusting the optical retardation ranges from zero to 50 nm and an optimum value of xcex2 for adjusting the optical retardation ranges from zero 50 nm.
In another aspect of the present invention, a transflective liquid crystal display includes upper and lower substrates facing and spaced apart from each other, wherein the upper and lower substrates include a plurality of pixel region that display images; an upper quarter wave plate (QWP) on the upper substrate; an upper half wave plate (HWP) on the upper QWP; an upper polarizer on the upper HWP; a lower quarter wave plate (QWP) under the lower substrate; a lower half wave plate (HWP) under the lower QWP; a lower polarizer under the lower HWP; a backlight device arranged adjacent to the lower polarizer; a liquid crystal layer interposed between the upper and lower substrates; a transparent common electrode on a surface of the upper substrate facing the lower substrate; an upper alignment layer between the transparent common electrode and the liquid crystal layer; a pixel electrode over the lower substrate, wherein the pixel electrode corresponds to each pixel region, and wherein the pixel electrode is divided into transparent and reflective portions; and a lower alignment layer between the pixel electrode and the liquid crystal layer; wherein a transmissive axis of the upper polarizer is perpendicular to a transmissive axis of the lower polarizer; wherein a slow axis of the upper QWP is perpendicular to a slow axis of the lower QWP; wherein a slow axis of the upper HWP is perpendicular to a slow axis of the lower HWP; wherein an optical retardation of the upper QWP is xcex/4+xcex1; wherein xcex1 ranges from zero to 100 nm; wherein the slow axis of the lower QWP is parallel with an orientation direction of the liquid crystal display layer; wherein an optical retardation of the lower QWP is xcex/4xe2x88x92xcex2; and wherein xcex2 ranges from zero to 100 nm.
An optical retardation of the liquid crystal layer is different between transmissive and reflective portions, wherein the optical retardation is xcex/4+xcex1in the reflective portion, and wherein the optical retardation is xcex/2+xcex1xcex2in the transmissive portion. An optimum value of xcex1 ranges from zero to 50 nm for adjusting the optical retardation and an optimum value of xcex2 ranges from zero to 50 nm for adjusting the optical retardation.
The slow axis of the upper HWP forms an angle of xcex8 with the transmissive axis of the upper polarizer. The slow axis of the upper QWP forms an angle of 2xcex8+45xc2x0 with the transmissive axis of the upper polarizer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.